For automatic vehicular transmission, it is common to control in a targeted manner the build-up and build-down of pressure at the setting mechanisms of the transmission elements, e.g., the band brakes, disk brakes, disk clutches, and the like, so that it is possible to adapt the pressure control to various operational conditions. For example, it is desirable that the control pressure for setting the action of brakes and clutches be varied in response to the entry and exit rotary speed relationships, to the rotational moment at entry, to the operational condition of the vehicle, etc. Thereby, the pressure level of the control pressure is set once according to a predetermined pattern to the automatic transmission, so that a conclusion can be drawn mathematically from the controlled pressure level to the rotational moment transferred in each case by the transmission element.
For the many uses and applications, this type of pressure control does not directly cause any serious problems, since, e.g., the rotation moment transformer installed ahead of the automatic transmission can, to a certain degree, buffer and compensate for imprecision in the pressure control. Nonetheless, with increasing length of operation of the automatic transmission, life-span problems of the transmission elements and/or undesirable or uncomfortable operational behavior of the drive can result from transmission element wear.
These problems can become especially palpable in connection with automatic transmissions without hydraulic rotation-moment transformers. In such cases, a part of the transmission also has to be used for startup. These transmission clutches have to be of considerably larger dimensions than the other transmission clutches in order to be able to absorb the quantities of transmission part heat which occur during a brief period of time during start-up. Also, the friction surfaces of these start-up clutches have to be designed as large as possible in order to achieve as low a specific heat load as possible.
These variously-designed transmission elements have the practical consequence that changes or fluctuations of pressure application affect the rotation-moment transmission of the various transmission elements to differing degrees, which can be seriously detrimental to the clutch control and to the comfort of the passengers.